Electroacoustical apparatus



Aug. 16, 1938. F. MASSA ELECTROACOUSTICAL APPARATUS 2 Sheets-Sheet 1 Filed Jun 30, 1936 nventor Fr an 7i Mas s a (Ittorneg Il. J:

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Aug; 16, 1938.

F. MASSA ELECTROACOUSTICAL APPARATUS Filed June Pl. HVE W19 VE 2 Sheets-Sheet 2 Fig/z :inventor Patented Aug. 16, 1938 uNiTEp STATES PATEN'H` OFFIEE Radio Corporation of Delaware Application June 30,

4 Claims.

This invention relates to electro-acoustical apparatus of the type disclosed in United States Reissue Patent No. 19,115 of Harry F. Olson and Julius Weinberger, and more particularly to the method of orienting the dead axis, or axis of minimum response of such apparatus.

In the aforesaid patent, there is disclosed a uni-directional sound pick-up device or microphone of the ribbon type which is operated 'in response to both the pressure of a sound wave and the pressure gradient, or particle velocity, thereof by terminating a part of the ribbon diaphragm in a pipe or tube. Such a microphone has the advantage that its response is substantially uniform throughout a comparatively wide angle.

In order that a microphone of this type may be used to best advantage under various conditions of use, however, it is essential that its axis of minimum response be so arranged or oriented that it will always extend from the microphone to the source or sources from which it is desired not to pick up any sound, while at the same time retaining maximum response in the direction of desired pick-up, and it is the primary object of my present invention to provide a method of accomplishing this result.

Another object of my invention is to provide a method of controlling not only the direction of shift of the dead axis, but also the magnitude of its shift.

According to my invention, I arrange the pressure actuated ribbon section and its associated pipe or tube in predetermined relation to the pressure gradient actuated ribbon section, and thereby control the direction of shift of the dead axis. For example, the pressure actuated section may be placed above the pressure gradient actuated section, in which case the dead axis will extend upwardly away from the microphone at the rear thereof, or the microphone ribbon sections may be so arranged that the pressure actuated one is below the pressure gradient, or velocity, actuated one, in which case the dead axis extends downwardly away from the microphone at the rear thereof, the ribbon sections in each case extending vertically. To control the magnitude of the axis shift, the size of the projecting pipe behind the pressure section may be changed, the longer the projection, the greater being the angle of shift.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims, but the invention itself, together with additional objects and ad- America, a corporation of 1936, Serial No. 88,124

vantages thereof, will best be understood from the following description when read in connection with the accompanying drawings in which Figure 1 represents the directional characteristic of a microphone of the type under consideration,

Figure 2 is a diagrammatic view illustrating a plane wave approaching from the rear of the microphone, y

Figure 3 is a Vector diagram of the voltage induced in such a microphone at low frequencies,

Figure 4 is a vector diagram of the voltage induced therein at high frequencies,

Figure 5 is a view similar to Fig. 2 showing the direction of the dead axis when the pressure actuated section is above the pressure gradient actuated section,

Figure 6 is a view similar to Fig. 5 but showing the direction of the dead axis when the pressure actuated section is below the pressure gradient actuated section,

Figure 7 is a sectional view through the microphone ribbon elements and the pipe behind the pressure actuated section showing the increase in path length to the front of the ribbon resulting from a pipe of predetermined cross-sectional dimensions behind the pressure actuated section,

Figure 8 is a view similar to Fig. '7 but showing a further increase in the path length to the front roi' the ribbon by the addition of a shell to the pipe behind the pressure actuated section,

Figure 9 is a set of curves showing the relation between the microphone response and the direction of a plane wave approaching from the rear of the microphone at various angles to the normal` axis of the microphone ribbon, at diierent frequencies, and with different path lengths, as illustrated in Figs. 7 and 8,

Figure 10 is a View similar to Fig. 5 indicating the direction in which the angle represented by L the abscissa of Fig. 9 is measured, that is, from an axis normal to the plane of the microphone ribbons at the rear thereof toward the pressure actuated section of the ribbons, and

Figure 11 is a table showing various values of the microphone response for specic angles represented by the abscissa of Fig. 9 at two different frequencies and for two conditions of path length difference as represented by Figs. '7 and 8.

The directional characteristic of a microphone of the type under consideration is represented by a cardioid of revolution, as shown in Fig. 1, withthe maximum response at the front side and zero response directly to the rear of the microphone on an* axis normal to the-surface of the ribbon.

The microphone response is represented by the equation E=K(1+cos where qs is the angle made from the axis of maximum response; K is a constant which is numerically equal to one-,half the maximum value of E; and E is the microphone sensitivity at any an'gle qi. This relation holds strictly for an inflnitesimally small microphone immersed in a plane wave. In an actual structure with finite dimensions, however, there will be a departure from the condition described by the above equation. The reason for this departure is due to a space phase shift between the sound reaching the pressure actuated ribbon and that reaching the pressure gradient, or Velocity, actuated ribbon at. the higher frequeIlClES.

This phenomenon will be evident from a consideration of Fig. 2 which shows a conducting element in the form of a ribbon constituted by a portion I which is substantially freely accessible to air vibrations on both its front and back, whereby it is responsive to the particle velocity in the sound wave, orl the difference between the pressure exerted thereon at the front and the back, and a second portion 3 which terminates in a pipe 5 at its rear surface, whereby the latter portion responds only to the pressure in the sound A plane wave 1 approaching from the rear' wave. of the microphone gives rise to a voltage E1 (Fig. 3) due to the particle velocity, or pressure gradient, component of the sound wave and a voltage E2 due to the pressure component of the sound wave. Under-ideal conditions, E1 would be equal in magnitude to E2 and opposite in phase, with a resultingzero `induced voltage in the microphone. Actually, however, due to the nite size of the pipe 5, the physical lengths of the paths from a sound source located at the rear of the microphone to the front of each of the portions I and 3 is different, that to the velocity actuated portion I being L1 and that around the pipe 5 to the front of the pressure actuated portion 3 y being somewhat longer than the path Lr and having a length L2.

At low frequencies, the difference in path length between the paths L2 and L1 is negligible compared to the wave length, and the voltages E1 and E2 aresubstantially in opposite phase as shown in the vector diagram in Fig. 3. At the higher frequencies, however, the difference between the lengths L2 and L1 is no longer a negligible partof the Wave length, and the voltage vector diagram becomes as shown in Fig. 4. Here, the voltage Ez is shifted by an angle 9 from its low frequency position, the magnitude of the angle 9 being given by the equation 6:21 I- radians where izthe'wave length of the sound at the particular frequency concerned. As a result of this phase shift, the combined voltage Eo is no longer zero, but is some nlte value, as shown in Fig. 4. It is apparent, therefore, that sound of relatively high frequency originating from some source X behind the microphone will induce a Voltage therein which will be transmitted to the reproducing system unless eliminated. This can be accomplished, at any predetermined frequency, by so orienting the dead axis, or axis of minimum response, that it will pass through the source X, in which case practically no sound emanating from the source X in the region of the aforesaid4 predetermined frequency will be picked up by the microphone.

I have found that the direction of the axis of -minimum response can be controlled generally by varying the relative positions of the portions I and 3 of the conducting element, and more speccally by varying the length of the path L2, as by varying the cross-sectional dimension of the pipe 5 immediately behind the ribbon portion 3 in a direction transverse to the ribbon. The portion I may, for example, be disposed either above the portion 3, as shown in Figs. 2 and 5, or it may be placed below the portion 3, as shown in Fig. 6, depending upon ythe conditions under which it is to be used.

When making talking motion pictures, for example, the microphone is usually suspended from the ceiling. For this purpose, it should. be arranged with the velocity actuated ribbon portion 3 above the pressure actuated portion I, as shown in Fig. 6, in which case the dead axis 9 extends downwardly away from the axis II which is normal to the microphone at the rear thereof. By adjusting the microphone to such a level that the dead axis 9 thereof passes through the camera (the point X in the drawings), it is clear that the camera noises will not be picked up by the microphone and therefore will not be recorded. Similar considerations prevail where the microphone is used in a public address System and the audience is below the level of the microphone and the speaker or other performer. On the other hand, if the audience is mainly abovethe level of the microphone, then it is desirable to arrange the microphone with the pressure actuated portion 3 above the velocity actuated portion I, `as shown in Fig. 5, whereupon the dead axis 9 will tip upwardly from the normal'axis Il as it leaves the microphone and the amount of noise picked up from the audience minimized. In general, therefore, the pressure actuated portion 3 of the conducting yelement should be placed at that end of the pressure gradient actuated lportion I which points in the direction in which itis desired that the dead axis shall tip or extend away from the normal axis Il.

'Ihe foregoing rule provides for controlling merely the direction of shift of the dead axis. The magnitude of shift may be controlled by varying the size of the projecting pipe 5 immediately behind the ribbon portion 3. This may be done, for example, by providing a pipe of larger external cross section transversely of the ribbon, or by adding sections onto the original pipe 5, or in any other suitable way, and the larger the projecting portions of the pipe 5, the longer will be the path length L2 and the greater the angle of shift.

In Fig. 7, there is shown a cross section of the pressure actuated ribbon element 3 of the microphone and the pipe 5 behind it. The path L1, along which a plane wave approaching the micro-l phone from the rear would travel to reach the element 3, were the pipe 5 infnitesimally small, is increased to a value L2 due to the finite size of the pipe 5. Figure 8 shows how the distance L2 is further increased by the addition of a shell 5a to the pipe 5 in order to increase the extent to which the pipe projects immediately behind and to the side of the ribbon portion 3. For any two condition of Fig. 7, in which L2 minus L1 has been assumed equal to one-quarter inch, and the point Il representing a similar minimum for the condition of Fig. 8 in which L2 minus L1 was assumed equal to one-half inch. In both cases, the separation between the centers of the elements I and 3 has been assumed to be 1 inch, as indicated in Fig` 10. 'I'he solid line curves A and B of Fig. 9 have been plotted for 4500 cycles, and the dash line curves C and D have been plotted for 2250 cycles. curves A, B, C and D are tangent at the points I 5 and I1 represents a true cardioid which would be the response characteristic of the microphone of Figs. 7 or 8 at extremely low frequencies where the wave length of the sound is considerably greater than the dimensions of the pipe.

From the foregoing description, it is believed that the advantages of arranging a uni-directional microphone in accordance with this invention will be fully apparent. Obviously, the microphone described is not limited in use to positions wherein the ribbon portions I and 3 are only in vertical alignment, as they may be horizontally aligned with equally satisfactory results. Many other changes will undoubtedly readily suggest themselves to those skilled in the art, and I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit ofthe appended claims.

I claim as my invention:

1. In a ribbon microphone of the type in which a portion of the ribbon terminates in a pipe and another portion is substantially freely accessible to air vibrations at its front and rear, the method of controlling the direction in which the axis of minimum response at a predetermined frequency extends which comprises orienting said axis by arranging, one of said ribbon portions in a predetermined relation to the other, and varying the angle between said axis and an axis normal to the plane of said ribbon by varying the size of said pipe behind said rst named ribbon portion.

2. In a ribbon microphone of the type in which a portion of the ribbon terminates in a pipe and another portion is substantially freely accessible The lower, solid line curve to which the.

to air vibrations at its front and rear, the method of controlling the direction in which the axis of minimum response at va predetermined frequency extends which comprises orienting said axis by arranging said rst named portion at that end of said second named portion which points in the direction in which it is desired to have said axis extend awaylfrom the microphone, and Varying the angle between said axis and an axis normal to the plane of said ribbon by varying the size of said pipe behind said rst named ribbon portion.

3. In a ribbon microphone of the type in which a portion of thevribbon terminates in a pipe at the rear thereof and another portion is substantially freely accessible to air vibrations at its front and rear, lthe method of controlling the direction in which the axis of minimum response at a predetermined frequency extends with respectto a sound source located at the rear of the microphone which comprises orienting said axis by arranging one of said ribbon portions in predetermined relation to the other, and varying the angle ybetween said axis and an axis normal to the plane of said ribbon by varying the path length for the sound from said source to the front of said first named ribbon portion.

4. In a ribbon microphone of the type in which a portion of the ribbon terminates in a pipe at the rear thereof and another portion is substantially freely accessible to air vibrations at its front and rear, the method of controlling the di- A rection in which the axis of minimum response at a predetermined frequency extends with respect to a. sound source located at the rear of the microphone 4which comprises orienting said axis by arranging said rst named portion at that end of said second named portion which points in the direction in which it is desired to have said axis extend away from an axis normal to the microphone, and varying' the angle between said axes by varying the path length for the sound from said source to the front of said first named ribbon portion.

FRANK MASSA. 

